The Universe Is Expanding Faster Than Expected, But New Results Raise More Questions

We've known for nearly a century that the Universe is expanding. The fact that galaxies are receding away from us was first demonstrated by Edwin Hubble in 1929, building upon the work of Henrietta Leavitt and others. Since then we've developed a variety of ways to measure the rate of cosmic expansion, and while they are broadly in agreement, there are small discrepancies between them. As a result we still don't know exactly how fast the Universe is expanding, as astrophysicist Ethan Siegel has so clearly explained. Now a new method of measuring cosmic expansion may settle the issue, but it also raises more questions.

It all comes down to a physical parameter known as the Hubble constant. The bigger Hubble constant, the greater the rate of cosmic expansion. The value of the constant also tells us the age of the Universe. If you trace the expansion backwards through time, you reach the point where the Universe was extremely hot and dense, commonly known as the big bang.

Hubble's original measurement of the constant compared the distances of galaxies with the redshift of their light. He calculated galactic distances by measuring the brightness of variable stars known as Cepheid variables, and combined them with measurements of galactic redshifts made by Vesto Slipher. He found that more distant galaxies had greater redshifts, and presumably were receding from us at a greater rate. Hubble's original value for the constant was about 500 km/s/Mpc, which caused a bit of a cosmological crisis. If the value was correct, the Universe was only about 2 billion years old, which contradicted geological evidence that showed the Earth was more than 4 billion years old.

Credits: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)

Over time our measurements of the Hubble constant got better, and the results settled around a much smaller value of around 70 km/s/Mpc, putting the age of the Universe at about 14 billion years. We also developed different ways to calculate the Hubble constant using different types of data, and they each produced similar results. This means we know for sure that the Universe is expanding, and we have a pretty good handle on just how fast it's expanding. But while these different methods broadly agreed, they didn't exactly agree. It was generally thought that as our measurements got better this discrepancy would go away, but it didn't. Something was clearly wrong with our understanding of cosmic expansion.

Hubble's method of comparing distance with redshift has been extended by shifting from Cepheid variables to supernovae. A particular type of supernova known as Type IA allows us to determine galactic distances across billions of light years. In 2016, observations from the Hubble telescope using this approach gave a value of 73.24±1.74 km/s/Mpc, which is on the high side of modern values.